This application relates to a gas turbine engine having a gear reduction driving a fan, and wherein exit guide vanes for the fan are provided with noise reduction features.
Gas turbine engines are known, and typically include a fan delivering air into a compressor section. The air is compressed and passed into a combustion section where it is mixed with fuel and ignited. Products of this combustion pass downstream over turbine rotors, and the turbine rotors are driven to rotate the compressor and fan.
Traditionally, a low pressure turbine rotor drives a low pressure compressor section and the fan rotor at the identical speed. More recently, a gear reduction has been provided such that the fan can be rotated at a reduced rate.
With this provision of a gear, the diameter of the fan may be increased dramatically to achieve a number of operational benefits. Increasing fan diameter, however, implies that the fan noise sources will dominate the total engine noise signature.
A major fan noise source is caused by rotating turbulent wakes shed from the fan interacting with the stationary guide vanes. The stationary guide vanes, or fan exit guide vanes are positioned downstream of the fan. A discrete, or tonal, noise component of this interaction is caused by the specific number of fan wakes interacting with the specific number of vanes in a periodic fashion. A random, or broadband, noise source is generated from the nature of turbulence inside each fan wake interacting with each guide vane.
At a given engine power condition, if the ratio of guide vanes to fan blades is lower than a critical value, the tonal noise is said to be “cut-on” and may propagate to an outside observer location, e.g. an observer location either in the aircraft or on the ground. If the ratio of guide vanes to fan blades is above the critical value, however, the tonal noise is said to be “cut-off.” Total engine noise may be dominated by tonal and/or broadband noise sources resulting from the fan wake/guide vane interaction.
Traditional acoustic design addresses tonal noise by targeting a cut-off vane count for subsonic fan tip speeds. The broadband noise, however, may require a lower vane count to decrease the number of turbulent sources. For a given number of fan blades, lowering the vane count below a critical value creates a cut-on condition and thus higher tone noise levels.
Thus, there is a tradeoff between addressing the two types of noise.
While cut-off vane counts have been utilized in the past, they have not been known in an engine including the above-mentioned gear reduction.